1. Field of the Invention
The present invention relates to an optical module to be used in fields of optical communications and optical information processing, for example, and more particularly, to an optical module for converting optical/electric signals into electric/optical signals by optically connecting an optical element having electric wirings with an optical fiber held by a ferrule and having a tip end portion without jacketing layers that is projecting from the ferrule.
2. Description of the Background Art
In recent years, in conjunction with a drastic increase in the amount of data communications, there is an increasing demand for a photo-detecting or light-emitting optical module for handling high speed optical signals with the optical signal speed of over Gbit/sec (Gbps), for example, and its realization at a cheaper cost. In view of this demand, there are many reports of a method for realizing an optical axis alignment of an optical element and an optical fiber by a mechanical positioning alone (the passive alignment method) by providing a V-groove on a silicon substrate. There are also reports of a technique for molding this optical element mounted silicon substrate together with lead frames.
In the conventional passive alignment method, in the case of processing high speed signals with the optical signal speed over 10 Gbps, it is known that the use of materials having a lower dielectric constant than silicon is advantageous because of the decrease of the parasitic capacity. However, this in turn requires signal line wirings with very minute intervals so that there is a problem that it becomes difficult to manufacture a mold package using lead frames which is advantageous for realization of low cost.
FIG. 1 shows a perspective view of a structure of a conventional optical module. This optical module is a photo-detecting optical module using an ordinary resin package with metallic lead frames, which is manufactured by mounting a photo-diode having an equivalent frequency characteristic as that of a photo-detecting optical module of the present invention shown in FIG. 7 to be described below and a pre-amplifier IC on a package made of a uniform resin material and having a shape in which metallic leads are independently projected into the lateral directions. Namely, in this conventional optical module, a frame 92 is attached on a plastic substrate 91 to enclose its surrounding, and a quartz glass V-groove substrate 93 is provided at an approximate center of the plastic substrate 91 inside this frame 92. A photo-diode 11 is implemented on this quartz glass V-groove substrate 93, and this photo-diode 11 and an optical fiber 5 are optically connected as a tip end portion 5a without jacketing layers of the optical fiber 5 abuts against this photo-diode 11.
The optical fiber 5 is fixed by being set in a V-groove 93a that is formed on the quartz glass V-groove substrate 93 along the optical fiber 5 underneath the optical fiber 5, and a portion projecting out from the quartz glass V-groove substrate 93 is held by a ferrule 7, where this ferrule 7 is fixedly held at a notch section 92a formed on the frame 92.
Also, a pre-amplifier IC 9 is provided adjacent to one end of the quartz glass V-groove substrate 93 on the plastic substrate 91, and this pre-amplifier IC 9 is electrically connected to the photo-diode 11 by bonding wires.
In addition, metallic lead frames 94 for pre-amplifier IC and metallic lead frames 95 for signals are provided on the plastic substrate 91, and one ends of these metallic lead frames 94 and 95 are electrically connected to the pre-amplifier IC 9 by bonding wires while the other ends are projected outside the plastic substrate 91.
In order to evaluate a receiving bandwidth of the conventional optical module in the above configuration, as shown in FIG. 2, this optical module was mounted on a glass epoxy frequency characteristic evaluation substrate 51 on which a plurality of electrode pads 52, signal lines 53 in a coplanar guide structure, and an SMA type high frequency connector 54 are formed, and they were electrically connected. Then, the receiving characteristic of the conventional optical module shown in FIG. 1 was evaluated in this connection configuration, to obtain the receiving bandwidth measurement result as shown in FIG. 3.
As can be seen in the receiving bandwidth evaluation result shown in FIG. 3, for this conventional optical module, large ripples which are probably caused by electrical reflections were appearing, and a large degradation of the frequency characteristic was observed. In this type of optical module, a characteristic impedance changes at a portion where the signal transmission leads are projected from inside the package to outside, so that the electrical reflections or resonances can occur easily and these electrical reflections or resonances can cause the degradation of the high frequency characteristic. In practice, an effort to prevent the degradation of the high frequency characteristic is made by cutting the signal line leads short and carefully implementing the optical module such that no gap is formed between a package of the optical module and the glass epoxy frequency characteristic evaluation substrate 51.
However, the wiring pattern of the glass epoxy frequency characteristic evaluation substrate 51 is usually formed at approximately 1 mm inner side of the edge as indicated by xe2x80x9cdxe2x80x9d in FIG. 2, so that this gap of 1 mm remains as a cause of the electrical reflections. For this reason, the large ripples that are probably caused by the electrical reflections were appearing and the large degradation of the frequency characteristic was observed as shown in FIG. 3.
FIG. 4 shows the conventional optical module of FIG. 1 implemented on an optical signal received waveform evaluation substrate which is manufactured by mounting a main amplifier IC 82 on a glass epoxy substrate 81 similar to the glass epoxy frequency characteristic evaluation substrate 51 of FIG. 2, in order to carry out the 10 Gbps optical signal received waveform evaluation with respect to a photo-detecting optical module using a resin package with metallic lead frames in a form of the conventional optical module of FIG. 1. Note that the main amplifier IC 82 is an amplifier having an automatic gain control (AGC) function of the operation frequency bandwidth approximately equal to 10 GHz, which outputs signals having a constant amplitude by uniformly amplifying the received signals as amplified by the pre-amplifier IC 9 used in the conventional optical module of FIG. 1. Using such an optical signal received waveform evaluation substrate, it is possible to output electric signals suitable for the bit error rate characteristic evaluation that is often used for the received waveform observation and the transmission characteristic evaluation.
FIG. 5 shows the evaluation result for the conventional optical module of FIG. 1 obtained by using the optical signal received waveform evaluation substrate of FIG. 4. In this evaluation, the received waveform outputted from the main amplifier IC 82 at a time of receiving optical signals modulated by 10 Gbps pseudo-random pattern (NRZ223xe2x88x921) with an average power of xe2x88x9210 dBm was observed using a wide bandwidth oscilloscope having a bandwidth of 20 GHz. FIG. 5 shows the observed waveform, which is a largely degraded waveform compared with the correct eye pattern. Also, when the bit error rate characteristic was evaluated in a vicinity of the average power of xe2x88x9210 dBm using the optical signal received waveform evaluation substrate of FIG. 4, it was impossible to realize a completely error free operation without any code recognition error.
Thus the conventional optical module package is associated with a problem that the implementation to connect signal line leads to an external substrate requires considerable cares in order to transmit the high speed signals with the optical signal speed over 10 Gbps to the external substrate without causing any degradation.
Also, the conventional optical module has a problem the characteristic impedance changes at a portion where the signal transmission leads are projected from inside the packet to outside, so that the electrical reflections or resonances can occur easily to cause the degradation of the high frequency characteristic, as can be seen from the received bandwidth evaluation result described above.
Also, the conventional optical module has a problem that it is impossible to realize a completely error free operation without any code recognition error as can be seen from the bit error rate characteristic evaluation in the optical signal received waveform evaluation described above.
It is therefore an object of the present invention to provide an optical module capable of connecting signal lines to an external substrate easily without causing any degradation of the transmission characteristic, and an optical module on which very minute wiring pattern can be realized.
According to one aspect of the present invention there is provided an optical module for converting optical/electric signals into electric/optical signals by optically connecting an optical element having electric wirings with an optical fiber held by a ferrule and having a tip end portion without jacketing layers that is projecting from the ferrule, the optical module comprising: an optical element mounting block having an optical element mounting section for mounting the optical element, and an optical fiber guide for guiding the optical fiber to align optical axes of the optical fiber and the optical element, such that the optical fiber is optically connected with the optical element while maintaining a state in which optical axes of the optical fiber and the optical element are aligned; and a wiring lead integrated resin substrate having a ferrule holding section for holding the ferrule, an optical element mounting block housing section for housing the optical element mounting block, and wiring leads to which the optical element is electrically wired; wherein at least a part of the wiring leads are provided by signal lines in a coplanar guide structure entirely formed on the wiring lead integrated resin substrate such that a characteristic impedance does not change when the wiring leads are electrically connected to external wirings.
Other features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings.